Toxic chemicals may be behind the rising number of children with autism, attention-deficit hyperactivity disorder and dyslexia, U.S. researchers say.

Co-authors Philippe Grandjean, adjunct professor of environmental health at the Harvard School of Public Health in Boston, and Philip Landrigan, dean of the Icahn School of Medicine at Mount Sinai in New York, said the study outlines possible links between newly recognized neurotoxicants and negative health effects on children, including:

-- Solvents linked to hyperactivity and aggressive behavior.

-- Certain types of pesticides might cause cognitive delays.

"The greatest concern is the large numbers of children who are affected by toxic damage to brain development in the absence of a formal diagnosis," Grandjean said. "They suffer reduced attention span, delayed development, and poor school performance. Industrial chemicals are now emerging as likely causes."

The study follows up on similar work conducted by the authors in 2006 that identified five industrial chemicals as "developmental neurotoxicants," or chemicals that can cause brain deficits.

The study offers updated findings about those chemicals and adds information on six newly recognized ones, including manganese, fluoride, chlorpyrifos and DDT (pesticides), tetrachloroethylene (a solvent) and the polybrominated diphenyl ethers (flame retardants).

The researchers forecast many more chemicals than the known dozen or so identified as neurotoxicants contribute to a "silent pandemic" of neurobehavioral deficits that is eroding intelligence, disrupting behaviors, and damaging societies.

But controlling this pandemic is difficult because of a scarcity of data to guide prevention and the huge amount of proof needed for government regulation, they said.

"Very few chemicals have been regulated as a result of developmental neurotoxicity," the researchers wrote in their report on their findings.